This invention relates, in general, to semiconductor devices and, more particularly, to a novel process for forming contoured openings in insulating layers of semiconductor devices.
It has long been known that there exists a need, in the manufacture of semiconductor devices, to round the corners of lower layers of a multi-layer structure so that when subsequent layers are deposited, the surface presented to the subsequent layer will not have sharp or abrupt steps for the subsequent layers to traverse. Deposition of the subsequent layers, without the prior removal of the undesirable contours, may result in discontinuities in the metal interconnect line, thus producing an inoperative device and lowering the overall yield. Further, since some lower or subsequent layers are conductive, that is, metallic or doped polycrystalline silicon (polysilicon) lines, these lines must be insulated from each other.
The deposition of glass insulating films that can be flowed at temperatures of, for example 1200-1300.degree. C. to produce a gradual taper over steep steps can usually alleviate this situation. In U.S. Pat. No. 3,833,919 which issued to C. T. Naber on Sept. 3, 1974, there is described a multi-level conductor structure using a lower, undoped silicon oxide insluating layer and a second layer of phosphorous doped oxide formed thereover. The doped oxide layer, when heated to about 1000.degree. C., will flow over the steps in the undoped oxide layer. However, there are difficulties associated with this procedure. For example, after contact openings are formed, the maintenance of exposed areas of silicon at elevated temperatures, for extended periods of time, will cause the formation of an undesirable oxide on the exposed silicon. Additionally, the exposed silicon is susceptible of being doped, by diffusion, from the doped oxide layer while the elevated temperature will drive the dopant from the heavily doped layer into the undoped layer. To prevent doping of the undoped layer, a layer of silicon nitride must be interposed between the two oxide layers. Then, in order to remove any oxide that may have been formed on the exposed silicon areas, an additional processing step will have to be introduced to etch the undesirable oxide which in turn will etch desirable oxide.
Accordingly, the deposition of a glass film that can be made to produce a gradual taper over steep steps in the substrate will alleviate this undesirable situation. Phosphosilicate glass (PSG) with about 6-8 wt % P has been used for this purpose since such glasses have been found to have good dielectric and sodium gettering properties and, additionally, can be readily formed, using chemical vapor deposition techniques, from the hydrides. However, such glasses are undesirable in that the fusion or flow temperatures are in the range of about 1000.degree.-1100.degree. C. which have been found to be too high to produce satisfactory radiation-hardened complementary MOS integrated circuits and other heat sensitive large scale integrated circuits. Increasing the phosphorous content of PSG, while lowering the flow temperature, nevertheless increases the chances of corrosion during operation of the device.
In U.S. Pat. No. 3,481,781, which issued to W. Kern on Dec. 2, 1969 and is assigned to the same assignee as the subject application and in an article "Chemical Vapor Deposition of Silicate Glasses for Use with Silicon Devices" by W. Kern et al., J. Electrochem. Soc.: ELECTROMECHANICAL TECHNOLOGY 117, Apr. 1970 (I Deposition Techniques, pp. 562-568) and (II Film Properties, pp. 568-573) there are the initial discussions of the use and method of forming borophosphosilicate ternary glasses (BPSG). These BPSG layers are especially attractive to the semiconductor art as it has been found that they are more compatible with positive photoresist in that a positive photoresist will adhere better to BPSG than negative photoresist. In either event, adhesion to BPSG layers is much better than to the prior art 6 wt % PSG. It has also been found that with BPSG, the constituents thereof may be tailored so that it flows at about 900.degree.-950.degree. C. and will initially soften at a temperature of about 750.degree. C.